1. Field of the Invention
The present invention relates to a device for flushing and aerating a surface coated with microorganisms in a plant for biological purification of waste water. The device comprises a rotor (worm) in the form of a spirally coiled channel which is rotatable about a horizontal axis above the surface of the waste water so that the inlet mouth of the channel receives air as well as waste water during its rotation, and is characterized in that the rotor has its central coil connected to the central coil of a corresponding rotor of opposite coil direction and mounted coaxially with the first one so that its outlet mouth is located below the surface of the waste water during part of its rotation, and in that both rotors are filled with a medium on which microorganisms can grow.
2. Description of the Prior Art
Biological purification of waste water can be effected by causing a medium, e.g. broken stones, plastic balls or the like, to be coated with microorganisms that consume nutrients, ammonia and the like, by irrigating it with or immersing it in waste water. This reduces the contents of such substances in the water.
A known rotary filter plant comprises a drum which contains a light medium, such as plastic rings or balls, and rotates with part of its periphery immersed in the waste water. This plant suffers from the drawbacks that the medium is insufficiently aerated and flushed in the interior of the drum, resulting in a poor utilization of the active surface of the medium and insufficient replacement of the coating of microorganisms. Moreover, the plant tends to freeze at low temperatures. Attempts have been made to overcome these drawbacks by using blowers and pumps, but this is complicated and energy consuming.
The Swedish Published Application No. 345 842 discloses a worm-shaped contact filter disc for biological purification of waste water. This disc consists of a spirally coiled channel rotatable about a horizontal axis above the surface of the water. During rotation the channel inlet is alternately filled with air and water at the periphery of the disc. Both air and water are guided to the outlet of the channel at the centre of the disc during rotation, to thereby provide effective contact between the surface of the channel and alternately air and water throughout the length of the channel. This admittedly brings about a favourable environment for anaerobic microorganisms everywhere in the channel, but the device is rather energy consuming since the weight of the water which is transported by the disc is not used to provide a torque. Here too, the coating of microorganisms is replaced relatively slowly, and the device easily freezes.
A device which can be used for filtrating and humidifying air is known from the U.S. Pat. No. 2,106,025. This device comprises a rotary system of two worms which are built together and interconnected at their central coils, said worms having their respective coils oppositely directed to one another and having part of the periphery immersed in water. During rotation one worm lifts quantities of water towards its centre, and the other worm then lowers the water to the starting level again. It is expressly stated in the patent that the water used for humidifying the air is polluted by the use of the device, and therefore the artisan will not be induced to consider using this device for purifying waste water, nor are such theoretical possibilities mentioned in the patent.